Refactor to directly use JSON output from Metamap2016

The newest version of [Metamap now supports JSON format](https://metamap.nlm.nih.gov/Docs/FAQ/JSON.pdf) - update the [Metamap](https://github.com/NanoNLP/medaCy/blob/master/medacy/pipeline_components/metamap/metamap.py) wrapper to directly parse this information. It currently gathers the XML and manually turns it into JSON - this will make the code much cleaner and potentially faster